This invention relates to a method of controlling a plurality of subprocesses in a distributed control system and more particularly to a method of controlling a plurality of subprocesses in a distributed control system such that the preprogrammed set of instructions of each process control computer in the distributed control system is identical when the control function of each process control computer is not identical.
In a ring-type distributed control system such as disclosed in U.S. Pat. No. 4,015,548 each process control computer in the distributed process control system controls the associated subprocess, a coal pulverizer and elevation of fuel firing equipment. The same process control computer monitors the operation of the associated coal pulverizer and burner elevation equipment from a safety perspective. An adjacent process control computer also monitors the operation of the coal pulverizer and burner elevation from a safety perspective. Field inputs necessary to ascertain the status of safety equipment are wired to input ports of both the controlling process control computer and the process control computer providing redundant safety backup. Likewise, outputs necessary to control the subprocess are wired to the process control computer providing the control function; outputs necessary to shutdown the subprocess are wired to the process control computer providing redundant safety backup.
In the distributed process control system, the process control computer providing redundant safety backup has the ability to shutdown the coal pulverizer and burner elevation equipment that it is monitoring from a safety perspective. In an energized-to-start, energize-to-stop control system, a failure or otherwise removal from service of the processor providing control of its associated coal pulverizer and burner elevation does not remove the associated coal pulverizer and burner elevation from service. The associated coal pulverizer and burner elevation remain in service until an unsafe operating condition is approached at which time the adjacent processor monitoring the coal pulverizer and burner elevation from a safety perspective will safely shutdown the coal pulverizer and burner elevation utilizing the outputs connected thereto. However, the processor providing redundant safety backup does not have the ability to control the coal pulverizer and burner elevation, but rather only permits the coal pulverizer and burner elevation to remain in service at the status quo.
The distributed control system disclosed in U.S. Pat. No. 4,015,548 required in addition to process control computers (32-48) in the distributed process control system, two additional process control computers (50-52) to perform the unit function of monitoring the entire distributed control system and placing into service or removing from service subprocesses controlled by the distributed process control system such that the combined output from the subprocesses of the distributed process control system matched the demand therefor. Of the two process control computers performing the unit function in U.S. Pat. No. 4,015,548, one provided the unit function while the other served as a redundant backup.
What is needed is a method of controlling a plurality of subprocesses in a distributed control system that eliminates the separate process control computer dedicated to the unit function and a separate process control computer providing backup therefor. Such a method of controlling a plurality of subprocesses in a distributed control system would then leave only process control computers in the distributed process control system wherein each process control computer could execute the same preprogrammed set of instructions as all of the other process control computers in the distributed control system even though the control function of each process control computer is not identical. This design would necessitate maintaining only one spare program or one spare process control computer.